In bushings, high demands are placed on the sealing between the conductor and the surrounding insulator body, so that no gas or liquid, for example transformer oil, may leak in the boundary layer between these.
WO 00/55872 discloses a bushing intended for connection to the wall of a transformer tank. The bushing according to the patent exhibits an insulator body 17 that is applied to a conductor 15. The patent relates to means to sealingly connect the bushing to the transformer housing. The problem with sealing between the insulator body and the conductor is not dealt with in the patent.
U.S. Pat. No. 3,775,547 discloses another example of a bushing exhibiting means integrated into the insulator body for connection of the bushing to a transformer housing. To solve the problem with sealing between the insulator and the conductor, it is proposed to attempt to adapt the coefficient of thermal expansion of the insulator material to the coefficient of thermal expansion of the conductor by supplying additives to the insulating material (column 3, lines 22 et seq.). The insulator body is here preferably made by casting and subsequent hardening of an epoxy material and is intended for lower voltages; a voltage level of 7 kV is, for example, mentioned in the patent. This solution of the leakage problem is not sufficient at the higher voltages to which the present invention relates.
In bushings for higher voltages, that is, above 36 kV and up to the highest system voltages occurring, 800 kV and above, difficulties of achieving satisfactory sealing arise since the size of the insulator body increases, which, among other things, in case of temperature variations, results in problems with sliding between the insulator body and the conductor due to the difference in the coefficient of temperature expansion of the material in the conductor, which usually consists of metal such as aluminum or copper or alloys thereof, and the material in the insulator. The contact between the insulator and the conductor may become released, which may then result in the occurrence of undesired leakage of gas/liquid.
It is common to apply a pressure-relieving layer, for example in the form of cork rubber, between the conductor and the insulator body. However, this layer does not ensure the sealing between the conductor and the insulator body, so the problems with leakage remain.
To ensure sealing, it is known to apply slots for seals, for example in the form of O-rings, at the end portions of the insulator. Such sealing measures are both complicated and costly in manufacture.